Infix_to_Postfix.c

// To convert an infix expression to an postfix expression

#include <stdio.h>
#include <string.h>
#include <stdlib.h>


struct Stack
{

	int top;
	unsigned capacity;
	int* array;

};


struct Stack* createStack( unsigned capacity )
{
	
	struct Stack* stack = (struct Stack*) malloc(sizeof(struct Stack));
	if (!stack)
 		return NULL;
	stack->top = -1;
	stack->capacity = capacity;
	stack->array = (int*) malloc(stack->capacity * sizeof(int));
	if (!stack->array)
		return NULL;
	return stack;

}

int isEmpty(struct Stack* stack)
{

	return stack->top == -1 ;

}


char peek(struct Stack* stack)
{

	return stack->array[stack->top];

}


char pop(struct Stack* stack)

{

	if (!isEmpty(stack))
		return stack->array[stack->top--] ;
	return '$';

}


void push(struct Stack* stack, char op)
{

	stack->array[++stack->top] = op;

}


int isOperand(char ch)
{

	return (ch >= 'a' && ch <= 'z') || (ch >= 'A' && ch <= 'Z');

}


int Prec(char ch)
{

	switch (ch)
	{
	case '+':
	case '-':
		return 1;
	case '*':
	case '/':
		return 2;
	case '^':
		return 3;
	}
	return -1;

}


int infixToPostfix(char* exp)
{

	int i, k;
	struct Stack* stack = createStack(strlen(exp));
	if(!stack) 
		return -1 ;
	for (i = 0, k = -1; exp[i]; ++i)
	{
		if (isOperand(exp[i]))
			exp[++k] = exp[i];
		else if (exp[i] == '(')
			push(stack, exp[i]);
		else if (exp[i] == ')')
		{
			while (!isEmpty(stack) && peek(stack) != '(')
				exp[++k] = pop(stack);
			if (!isEmpty(stack) && peek(stack) != '(')
				return -1;
			else
				pop(stack);
		}
		else 
		{
			while (!isEmpty(stack) && Prec(exp[i]) <= Prec(peek(stack)))
				exp[++k] = pop(stack);
			push(stack, exp[i]);
		}

	}
	while (!isEmpty(stack))
	exp[++k] = pop(stack );
	exp[++k] = '\0';
	printf( "%s", exp );

}


int main()

{

	char exp[100];
	printf("Enter the Infix Expression: \n");
	scanf("%s",exp);
	infixToPostfix(exp);
	return 0;

}